crucible-agent needs to do more at the same time

[leftwo]

Right now, the worker loop in crucible agent is single threaded.
Creating a region from a clone source can take a while, and this can backup
other agent commands that could be independent. Figure out a way to
make the agent do multiple things at once to prevent a large clone operation
from making a backlog of other commands.

[leftwo]

With snapshot replacement about to land, this will become more visible.

[leftwo]

Proposed fix in: #1393